When a mentor is suspended due to inactivity, we must make sure that they won't re-enter immediatly mentorship while they are still not available and hence would be automatically re-suspended for inactivity rapidly. This would be counter-productive for their mentees.

I don't understand. The main issue for me, I think, is what you meant by "re-enter mentorship"—what does that mean? Wouldn't it simply mean responding to some user's question? And if that is what it means, they they are available, because they responded, so there seems to be a self-contradiction, and I don't really follow. Or does "re-entry" (I would probably say 'resumption of...' or 'resume...' membership) mean something else?

Or do you mean perhaps something like enforcing a minimum suspension, so once they are automatically suspended, they cannot respond to questions during interval X? What if the auto-suspend happens on day 10 of inactivity for minimum 30 days, but they are actually back from vacation on day 12? Can they then 'apply' for early reinstatement of mentorship?

Can an auto-suspended mentor see to which Mentor their mentees' questions are being diverted to? If not, why not? If so, what happens if suspended mentor M1 follows his mentee questioner Mq1 over to on-call mentor M2's page , and responds to Mq1's question at M2's talk page? (That's what I might do.) Is M1 automatically reinstated? Or are they just in mentorship jail for the duration?

When a mentor is suspended, they are marked as away, the same way the'd mark themselves as away, or an admin park them as away. All questions to them are dispatched to other mentors -- plural, as the goal to prevent other mentors being overwhelmed. You can resume mentorship by marking you as active in your Mentor Dashboard. By doing this, you will get new questions from your assigned mentees (the conversations they started with another mentor remain active at that mentor's talk page) and new mentees are assigned to you.

I don't understand. The main issue for me, I think, is what you meant by "re-enter mentorship"—what does that mean?

I got that same question privately, and re-reading my sentence I realize that it is confusing. :)

Some users volunteer for many things but forget that it comes with a commitment. For mentorship, the minimum is to respond to mentees' questions. Two cases are possible:

• you are busy and away from the wikis, and you won't return actively soon. However, you check on notifications from time to time. Getting a notification saying that you are suspended from mentorship could have two effects:
  • you acknowledge and remove yourself (or get removed)
  • you are afraid of loosing the role. So you resume your activity, and, as you are still busy, rapidly you stop. This behavior isn't really productive. I'm afraid that the notification would have this effect.
• you are busy but active, and you select editing above mentoring. This is another case documented at T429354: Identify and remove mentors who are active but who don't answer their mentees' questions

Can an auto-suspended mentor see to which Mentor their mentees' questions are being diverted to? If not, why not?

I don't think this was ever considered or suggested.

My take on this is that I trust other mentors to take care of my mentees, but I can understand the idea of checking if everything goes/went well. I do this by checking on the mentor dashboard, filtering questions on talk pages from my mentees but I understand that not everyone does this.

If so, what happens if suspended mentor M1 follows his mentee questioner Mq1 over to on-call mentor M2's page , and responds to Mq1's question at M2's talk page? (That's what I might do.) Is M1 automatically reinstated? Or are they just in mentorship jail for the duration?

Anyone can participate to a discussion. The way T321509: EPIC: Provide a system to automatically suspend or remove inactive mentors from the list of mentors is designed:

• marks users as away after X days of inactivity (0 edits) -- to me it implies that resuming editing stops you from being away (@Urbanecm_WMF to confirm)
• remove users as away after X days of inactivity (0 edits)